Heavy oils and bitumens make up an increasing percentage of available liquid hydrocarbon resources. As the demand for hydrocarbon-based fuels has increased, a corresponding need has developed for improved processes for desulfurizing heavy oil feedstreams. Processes for the conversion of the heavy portions of these feedstreams into more valuable, lighter fuel products have also taken on greater importance. These heavy oil feedstreams include, but are not limited to, whole and reduced petroleum crudes including bitumens, shale oils, coal liquids, atmospheric and vacuum residua, asphaltenes, deasphalted oils, cycle oils, FCC tower bottoms, gas oils, including atmospheric and vacuum gas oils and coker gas oils, light to heavy distillates including raw virgin distillates, hydrocrackates, hydrotreated oils, dewaxed oils, slack waxes, raffinates, and mixtures thereof.
Hydrocarbon streams boiling above 430° F. (220° C.) often contain a considerable amount of large multi-ring hydrocarbon molecules and/or conglomerated association of large molecules containing a large portion of the sulfur, nitrogen and metals present in the hydrocarbon stream. A significant portion of the sulfur contained in these heavy oils is in the form of heteroatoms in polycyclic aromatic molecules, comprised of sulfur compounds such as dibenzothiophenes, from which the sulfur is difficult to remove.
Processing of bitumens, crude oils, or other heavy oils with large numbers of multi-ring aromatics and/or asphaltenes can pose a variety of challenges. Conventional hydroprocessing methods can be effective at improving API for a heavy oil feed, but the hydrogen consumption can be substantial. Conversion of the liquid to less valuable products, such as coke, can be another concern with conventional techniques.
One general method in the art for hydroprocessing of heavy oils by treating heavy oils with alkali metal salts is exemplified in U.S. Pat. No. 4,003,823. Here, is disclosed a process for treating heavy carbonaceous feeds in the presence of an alkali metal hydroxide and hydrogen to convert the heavy feed into lighter (higher API) hydrocarbon products. In a similar manner, U.S. Pat. No. 4,003,824 discloses a process for treating heavy carbonaceous feeds in the presence of an alkali metal hydride and hydrogen to convert the heavy feed into lighter (higher API) hydrocarbon products. In both of these processes, coke (solid, high carbon materials) are produced during the reactions and are present as contaminants in the products derived from such processes.
In these processes, the coke typically either ends up in the separated product oils or in the separated spent alkali metal salts stream. In either case, the coke must be removed from at least one of these product streams and in either case is difficult and expensive to facilitate such selective removal of the coke contaminants. Neither of these patent disclosures addresses methods for removing the coke from the product streams although such removal is necessary for operating an efficient and cost effective process. As such, it is generally recommended therein to limit the severity (i.e., conversion rate) of the process and thus minimize the coke in the products. This however, lowers overall throughput or reduces the amount of product produced by limiting the overall process conversion. Another manner in which to reduce coke make is to add excessive hydrogen (at higher hydrogen partial pressures) which will tend to reduce the total coke make in the process. However, the use of added/excess hydrogen is very costly and has limited benefits, and as such, is practice which is to be avoided by refiners if possible.
U.S. Patent Application Publication Nos. 2011/0147273 and 2011/0147274 also disclose processes for conversion of heavy hydrocarbon feeds into lower molecular weight products in the presence of alkali metal salts and hydrogen as well as processes for regenerating the spent alkali metal salts. However, similarly, the processes described in these applications do also result in coke being formed in the product streams and do not address efficient methods for removing such coke byproduct contaminants from either the oil or alkali metal product streams. Similar to as discussed above, it is recommended to minimize the coke produced in the process by limiting the severity of the process (including limiting the amount of time subjected to the reaction conditions) as well as utilizing a two-stage reactor system to allow processing flexibility for minimizing the coke produced in each stage.
What is needed in the industry is a process for effectively removing the coke products from alkali metal salts hydroconversion processes that is simple and inexpensive, thus allowing the hydroconversion reactions to be run at higher severity in order to produce higher amount of converted products with the same major process equipment.